On the internal structure of starless cores

II. A molecular survey of L1498 and L1517B

¹ Observatorio Astronómico Nacional, Alfonso XII 3, 28014 Madrid, Spain e-mail: m.tafalla@oan.es
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
³ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands

Received: 29 March 2006
Accepted: 12 May 2006

Abstract

Context.Low mass starless cores present an inhomogeneous chemical composition. Species like CO and CS deplete at their dense interiors, while N₂H⁺ and NH₃ survive in the gas phase. As molecular line observations are used to determine the physical conditions and kinematics of the core gas, chemical inhomogeneities can introduce a serious bias.

Aims.We have carried out a molecular survey towards two starless cores, L1498 and L1517B. These cores have been selected for their relative isolation and close-to-round shape. They have been observed in a number of lines of 13 molecular species in order to determine a self-consistent set of abundance profiles.

Methods.In a previous paper we modeled the physical structure of L1498 and L1517B. Here we use this work together with a spherically-symmetric Monte Carlo radiative transfer code to determine the radial profile of abundance for each species in the survey. Our model aims to fit simultaneously the radial profile of integrated intensity and the emerging spectrum from the core center.

Results.L1498 and L1517B present similar abundance patterns, with most species suffering a significant drop toward the core center. This occurs for CO, CS, CH₃OH, SO, C₃H₂, HC₃N, C₂S, HCN, H₂CO, HCO⁺, and DCO⁺, which we fit with profiles having a sharp central hole. The size of this hole varies with molecule: DCO⁺, HCN, and HC₃N have the smallest holes, while SO, C₂S and CO have the largest holes. Only N₂H⁺ and NH₃ seem present in the gas phase at the core centers.

Conclusions.From the different behavior of molecules, we select SO, C₂S, and CH₃OH as the most sensitive tracers of molecular depletion. Comparing our abundance determinations with the predictions from current chemical models we find order of magnitude discrepancies. Finally, we show how the “contribution function” can be used to study the formation of line profiles from the different regions of a core.